Focal Spots Distribution Characteristics Of Lasers Projecting In Hohlraum Wall In Direct Driven

For the center-core ignition model of indirect-drive laser inertial confinement fusion, uniform irradiation of light intensity on the hohlraum wall is one of the critical conditions for a successful ICF ignition. At the same time, a uniform irradiation of the target wall partially improves the pellet compression ratio, and therefore the requirement of laser energy is reduced accordingly. Though the cross section of the injected beams shares the same profile, many other factors affect the irradiation uniformity on the target wall such as the number of laser beams, the spatial arrangement of laser beams, the intensity distribution, focusing position and perfection degree of the hohlraum wall and so on. In the initial stage of laser ablation, the X-ray radiation characteristics on the spherical fuel pellet is influenced to a certain degree by the uniformity of laser projecting on cylindrical target cavity wall. Therefore, it is necessary to research in detail the focal spot intensity distribution of laser on the target wall, which is previously focused by the lens before entering the target chamber.In consideration of the importance of a uniform distribution on hohlraum wall,the main research contents in this thesis are listed as follows: the influence of the cluster modulation on the incident flattened Gaussian optical field in indirect-drive device, the intensity distribution of multi-flat-topped beams exposing in target and its wall, and the intensity distribution characteristics of partially coherent flat-topped beam group projecting in the cavity wall.In the high power indirectly driven fusion, based on the generalized Huygens-Fresnel diffraction integral formula, the light intensity distribution of the flattened Gaussian beam after the lens is derived under two conditions,one is the ideal case and the other is limited by a finite number of small size of "defect" in amplitude modulation. Based on the established light intensity model of the flattened Gaussian beam with cluster modulation, both the influence of the defocus amount and the incident angle of the ideal flattened Gaussian beams, and the influence of amplitude modulation, "defect" size, cluster modulation of the amplitude-modulated beam after lens focusing on the evolution regularity of light field on hohlraum wall are emphatically analyzed. The results show that for appropriately chosen values of defocus amount and incident angle, more uniform distribution of light intensity on the hohlraum wall is achieved. Different modulation sizes, modulation depths have the same effects on the transmission process of Gaussian beams, and with higher modulation depth, the resulting oscillation phenomenon of light intensity becomes more obvious.Analytical formulas are given to calculate the light intensity with focal spot coherent superposition for multi-beam flattened Gaussian laser beams projected in the interior and on the hohlraum wall that is applied in the facility of indirect-drive laser inertial confinement fusion. Analyses are emphasized on the influence of axial distance, the amount of defocus, angle of incidence and number of factors F on the projection light intensity distribution spots inside the cavity in cross section, and the influence of the defocus amount, angle of incidence, F number and size of the cavity diameter on the light intensity distribution of focal spot on the irradiation target wall. The simulation results show that with the increment of defocus amount and F number, the light intensity variation of the focal spot projected on the wall and the cross sections perpendicular to the axis of hohlraum becomes gentle, and the uniformity of the light intensity distribution becomes poor. With the increased focus or F number of defocus amount, the focal spot size presents a decreasing trend. Moreover, uniformity of beam spots in hohlraum wall is greatly relevant to hohlraum diameter and incident angle.A model was established to study the propagation process of the lens-focused incoherent laser beams in the hohlraum. The distribution characteristics of light intensity from the incoherent superposition intensity of focal spots is investigated for all laser beams in cone rings projected on the wall of hohlraum are studied based on Collins formula. In this work, the effects of the relative spatial correlation length, lens focusing F number and defocus amount on the intensity distribution uniformity of focal spot in target wall are also discussed in detail. With the increment of spatial correlation length and focusing F number, the increasing spots light intensity of incoherent superposition and light intensity fluctuation becomes more apparent as well. However, the uniformity of light intensity distribution for the top area deteriorates. For a larger defocus amount, the average light intensity and the fluctuation extent on the wall of hohlraum decreases, and the difference between the peak intensity become smaller, which makes the top light intensity distribution more uniform.